Indian Bend Wash

Superfund Site | South Indian Bend Wash: Page 2 of 2

The MAU lies below the UAU and located approximately 170 to 200 feet bgs, and consists primarily of clay and sandy silt with significant interbedded layers of sand and gravel mixtures. These coarser-grained interbedded layers generally represent the zones with higher hydraulic conductivity in the MAU. Weak to strong calcium carbonate cementation is also present in the MAU.  The groundwater flow direction in MAU sub-unit B is generally west to east, but insufficient data exist to fully characterize the flow direction. The groundwater flow direction in MAU sub-unit C varies from due north to east, with northeast appearing to be the predominant flow direction.

The lower alluvial unit (LAU) underlies the MAU and usually encountered at 500 feet bgs. Observations of the LAU indicate that the composition of the LAU is a conglomerate, dominated by weakly cemented gravel, sand, silt, and rock fragments. Limited data exist to estimate groundwater flow directions in the LAU. The general flow direction is to the east or northeast, similar to the MAU.

Action Taken

In 2016, the Army Corps of Engineers, on behalf of the U.S. EPA, completed the second Five Year Review (FYR). The FYR concluded that while the remedies are still protective of groundwater, indoor air near where soil was contaminated will need to be re-evaluated.

In April 2017, the U.S. EPA released a fact sheet explaining what vapor intrusion is, how it is determined if vapor intrusion is a concern, and the areas to be re-evaluated.

Status

Vapor intrusion work is currently underway at the SIBW Superfund Site, lead by the U.S. EPA. For more information on vapor intrusion, please contact the U.S. EPA.

An amended FYR will be released after the vapor intrusion work has been completed and evaluated.